   (fn [multidatasetable {:keys [format] :as opts}]
     (when (:sheet opts)
       (throw (IllegalArgumentException. "open-datasets cannot open a single sheet.  Use open-dataset* to do this.")))
-    (or format
-        (extension multidatasetable))))
+    (format-or-type multidatasetable format)))
 
 (defn open-datasets
   "Opens a lazy sequence of datasets from a something that returns multiple

        (.substring 1)
        keyword)))

(defmulti open-dataset*
  "Opens a dataset from a datasetable thing i.e. a filename or an existing Dataset.
The multi-method dispatches based upon a :format option. If this isn't provided then

:ext - An overriding file extension (as keyword) to force a particular
       file type to be opened instead of looking at the files extension."

  format-or-type)

(defmethod open-dataset* Dataset [dataset opts]
  dataset)
